This invention relates in general to filters useful in imaging systems such as ultrasound imaging and display systems for displaying such images, and more particularly to a system for filtering and displaying ultrasound Doppler imaging information on moving objects, such as blood flow or moving tissue.
Brightness mode or B-mode imaging is one of the most common type of ultrasound imaging. A B-scan is a view of a cross-sectional slice through an object. A narrow pencil beam of ultrasound is swept through a region of interest to define the scan plane. The beam is formed from bursts of ultrasound and a repetition rate of ultrasound pulse generation is selected. As the pulse propagates into the body along any scan line, echoes are generated which travel back to the receiver. These echoes vary in intensity according to the type of tissue or body structure causing the echoes. This data is presented on a display in which the brightness depends on the echo strength. The B-mode image is typically presented in a grey-scale of black and white. Frequently, B-mode imaging is used to display stationary and moving tissues in the human or animal body.
In addition to B-mode, ultrasound imaging may also offer two-dimensional Doppler flow detection, also known as color Doppler imaging on moving objects in the body, such as blood. As disclosed, for example, in U.S. Pat. Nos. 5,014,710; 5,165,413; and 5,285,788 assigned to Acuson Corporation of Mountain View, Calif., in addition to the ultrasound beam for B-mode imaging, a separate acoustic Doppler beam is steered at preselected angles and is optimized for Doppler data acquisition of moving objects such as blood. Information such as velocity, variance of velocity, and energy of the moving object may be acquired by analyzing the Doppler shift of ultrasound frequencies detected by the receivers from multiple sample volumes along the direction of each Doppler beam. The Doppler information acquired along multiple scan lines is displayed as a color-encoded image that is spatially coordinated with, superimposed upon, and simultaneously displayed with, the B-mode grey-scale image to facilitate clinical diagnosis, for example. The three patents referred to above are incorporated herein by reference in their entirety.
As described in U.S. Pat. No. 5,014,710 to Maslak et al., the Doppler information signals are passed through a high pass filter which eliminates the static B-mode information. The mean velocity for each sample is determined in a velocity estimator and then stored in a frame memory. The velocity estimator will typically include fast Fourier transform or autocorrelation circuitry and, typically, will compute other blood flow parameters including variance and power. The above-described system of Maslak et al. has been adopted in, for example, the Acuson 128 color Doppler subsystem.
When used for imaging blood flow, Doppler signals are derived from ultrasonic echoes from red blood cells and other moving objects in the blood vessels in a scan imaging format. These echoes are mixed with background noise which degrades the image. To improve the signal-to-noise ratio (SNR), it may be desirable to employ temporal persistence in the Doppler imaging system, where data is filtered in the temporal dimension to improve the signal-to-noise ratio, thereby trading temporal resolution for more "accurate" flow presentation. In other words, two or more consecutive frames of Doppler data may be filtered by computing an average value of these frames and the average value is displayed instead of the individual consecutive frames.
In one approach, the above temporal filtering is performed by first calculating the autocorrelation values of the Doppler data of successive frames. Then temporal filtering is performed in autocorrelation space. In order to maintain dynamic range and precision, the autocorrelation values may be maintained to a large number of bits during the temporal filtering operations. Since these operations must be performed at high speed in a real time system, this can be a costly approach. Retaining data with greater precision also requires large data storage which is also costly.
After temporal filtering, the filtered Doppler frames are then sent to a discriminator where the filtered Doppler data are compared to a certain threshold. If the filtered signal amplitude exceeds the threshold, then the signal is scan converted and then displayed. Otherwise, the pixel value corresponding to the signal to be displayed will be set so that no image such as a color image is displayed.
In another approach to temporal filtering, the autocorrelated Doppler data are sent to the discriminator for comparison to the threshold first without filtering and the output data frames from the discriminator are then filtered. While this approach avoids having to store or operate on autocorrelation values of large number of bits, such approach is disadvantageous since significant information may be lost in the discrimination process in the discriminator before temporal averaging, thereby degrading the subsequent temporal averages obtained.
None of the above-described systems is entirely satisfactory. It is therefore desirable to provide an improved temporal filter which avoids the above-described problems.